Foods such as popcorn, pizza that can be heated and, in the case of popcorn, popped quickly in the microwave oven have achieved an astounding degree of popularity in recent months. It is only necessary to place the package in the microwave oven for three or four minutes to obtain a piping hot bag of freshly popped popcorn or a serving of pizza.
Many commercially available heating packages contain a heater or susceptor formed from a thin sheet of a plastic resin such as a polyester film vapor-coated with a semiconductive layer, e.g. aluminum. The coated film is usually bonded to a support such as paper or paperboard. These susceptors require several components and extra processing steps which adds to the complexity and cost of production. Vapor coating and laminating the plastic sheet to the paper or paperboard backing makes the composite structure relatively expensive and requires a substantial investment in processing equipment.
It has also been proposed to coat a paper support sheet with a carbon-containing ink. For example, U.S. Pat. No. 4,518,651 describes the coating of 18-point board with carbon black suspended in a matrix of a polyester copolymer dissolved in an organic solvent to which is pressed a polyester film under a pressure of 1,250 psi for three minutes. All samples required the application of a substantial amount of pressure in order to be an effective heater U.S. Pat. No. 4,264,668 describes the use of a carbon strip for sealing bags. Carbon particles in the form of powdered coal or carbon black are mixed with a binder such as an acrylate-type binder. Several layers are printed one on top of the other in order to obtain an appropriate resistivity of approximately 6,000 ohms. The carbon black layer is used to seal polyethylene plastic to form two narrow sealed bands when held against a sealing device, e.g. a high frequency transformer that produces a high frequency electric field. The coated sheets are not used for heating in a microwave oven. Once the finished package is formed, no further heating occurs. Moreover, the carbon strips on the edge of the bag are not located appropriately to heat the contents of the package.
The carbon ink formulas described in the foregoing patents were tried but they did not provide uniform heating. Heating was usually accompanied by scorching, popping, arcing and burning. Moreover, the use of high pressure to form a product is unacceptable.
It was found, for example, that when carbon was used alone with a standard ink vehicle burning and uncontrolled temperature rise would occur with a likelihood of burning the package and the product being heated. It was also found that when carbon was mixed with an acrylic vehicle the resulting susceptor would burn up a package in about one minute. The package would start to brown at about 400.degree. F and quickly thereafter burn which is, of course, unacceptable. Once the package begins to carbonize, this facilitates further heating and accelerates the burning reaction which causes burning to occur at a faster rate. This can be referred to as runaway heating.
An important objective of the invention is to provide a microwave susceptor coating applied at normal pressure as an ink which, upon exposure to microwave heating, will produce a uniform heat without unacceptable arcing, popping, sparking or burning. It is another objective to obtain uniformity of heating in different portions of the package and also from one sample to another. The heating composition must be able to be coated, preferably by means of a conventional printing press directly onto a backing such as paper, paperboard or the like without the requirement for multiple superimposed printed coatings, plastic sheets or high pressure which increase production costs and capital requirements.
To be useful the susceptor composition must have all the qualities of a good printing ink including the proper rheological properties: viscosity, dilatency and thixotropy to avoid problems such as misting, splattering or dripping from freshly printed surfaces moving at high speed and must also transfer easily from the supply roll to the printing roll. While the compositions suited for the present invention can be printed by means of a variety of printing processes such as offset, intaglio, letterpress and the like, it is particularly desirable for the composition to be applied by flexography or gravure printing. The microwave interactive heating composition, which for convenience will hereinafter sometimes be referred to as ink, must also have other desirable ink qualities such as good film strength, rub resistance, flexibility, the ability to dry quickly, and should be nontoxic both in the pressroom and as a finished coating. It should also have good covering characteristics and be sufficiently stable to resist settling or other degradation during use. It should produce coatings of uniform thicknesses and be able to form a continuous or interrupted coating, e.g. a coating with a multiplicity of openings or uncoated spots within a coated area.